1. Field of the Invention
The present invention relates to loadbreak connectors and more particularly to improvements in loadbreak connectors which prevent flashover upon switching (opening) the loadbreak connectors.
2. Description of the Prior Art
Loadbreak connectors used in conjunction with 15 and 25 KV switchgear generally include a power cable elbow connector having one end adapted for receiving a power cable and another end adapted for receiving a loadbreak bushing insert. The end adapted for receiving the bushing insert generally includes an elbow cuff for providing an interference fit with a molded flange on the bushing insert. This interference fit between the elbow cuff and the bushing insert provides a moisture and dust seal therebetween. An indicator band may be provided on a portion of the loadbreak bushing insert so that an inspector can quickly visually determine proper assembly of the elbow cuff and the bushing insert.
The elbow cuff forms a cavity having a volume of air which is expelled upon insertion of the bushing insert. During initial movement of the loadbreak connectors in the disassembly operation, the volume of air in the elbow cavity increases but is sealed off at the elbow cuff resulting in a decrease in pressure within the cavity. The dielectric strength of the air in the cavity decreases with the decrease in air pressure. Although this is a transient condition, it occurs at a critical point in the disassembly operation and can result in dielectric breakdown of the opening interface causing a flashover or arc to ground. The occurrence of flashover is also related to other parameters such as ambient temperature, the time relationship between the physical separation of the connectors and the sinusoidal voltage through the loadbreak connectors.
Another reason for flashover while switching loadbreak connectors, prior to contact separation, is attributed to a decrease in dielectric strength of the air along the interface between the bushing insert and the power cable elbow to ground. As earlier described, a decrease in air pressure is momentarily formed by the sealed cavity between the elbow cuff and the bushing insert flange. The lower pressure in the cavity reduces the dielectric strength of the air along the connection interface possibly resulting in flashover.
Accordingly, it would be advantageous to design a loadbreak connector system including a power cable elbow and a loadbreak bushing insert which reduce or prevent the possibility of a flashover upon switching of the connectors.
It is an object of the invention to provide loadbreak connectors, which upon disassembly under load, prevent flashover from occurring at the interface of the connectors.
It is a further object of the invention to provide a power cable elbow connector and loadbreak bushing insert having a modified interface which is vented to prevent a decrease in air pressure therebetween and a resulting decrease in dielectric strength of the air causing a flashover.
It is still a further object of the invention to provide a power cable elbow connector and loadbreak bushing insert having an indicator band formed on the bushing insert and which is vented to prevent a decrease in air pressure therebetween and a resulting decrease in dielectric strength of the air causing a flashover.
It is yet another object of the present invention to provide a power cable elbow connector and a loadbreak bushing insert in which the distance from the energized electrode of the elbow to the ground electrode of the bushing insert is increased to avoid flashover.
It is still a further object of the present invention to provide a power cable elbow connector having an electrode or probe in which a portion of the electrode is covered with an insulating material to increase the flashover distance to ground.
It is yet another object of the present invention to provide a power cable elbow connector in which the bushing insert receiving opening includes, at its upper end, an insulating material positioned within the conductive insert portion of the elbow connector to thereby increase the distance between an energized electrode and ground.
In accordance with one form of the present invention, the loadbreak connector assembly includes a power cable elbow having a conductor receiving end and a loadbreak bushing insert insertion end and a loadbreak bushing insert. The loadbreak bushing insert includes an insulative outer housing having an axial bore therethrough, a conductive member positioned within the axial bore of the housing and wherein the outer housing is formed in three sections. The first end section is dimensioned to be seated in a universal bushing well, a second end section is dimensioned for insertion into the power cable elbow connector and the third section is a mid-section which is radially larger than the first and second end sections. The mid-section preferably includes a conductive portion for attachment of a ground conductor and a transition shoulder portion between the second end section and the mid-section. In order to prevent a pressure drop in a cavity formed between an elbow cuff of the elbow connector and the mid-section of the bushing insert, the transition shoulder portion of the bushing insert includes means for venting an annular top surface of the transition shoulder portion with the longitudinal side surface of the housing mid-section.
The venting means may be formed in a number of different ways including at least one vent groove formed in the transition shoulder portion of the outer housing, at least one through hole from the annular top surface to the longitudinal side surface, a circumferential groove formed in a transition shoulder portion, or a plurality of raised ribs circumferentially spaced along the transition shoulder portion of the outer housing. Furthermore, the cavity formed between the elbow cuff and bushing insert transition shoulder portion may include an elastomeric flap which fills the cavity therebetween preventing any pressure drop in the cavity.
In one embodiment, the venting means is included on an elbow seating indicator band formed on the transition shoulder portion of the bushing insert. Upon proper mating of the elbow to the loadbreak bushing, the indicator band is completely hidden from view under the elbow cuff. The transition shoulder portion is formed with a step or recess and the indicator band, molded or extruded of a contrasting bright color is placed in the step or recess. Thus, the band serves the dual purpose of indicating proper assembly of the elbow cuff and the bushing insert while also providing venting for the cavity formed therebetween.
Alternatively, the combination of a power cable elbow and loadbreak bushing insert may include a means for increasing the distance from an energized electrode to ground in order to prevent flashover during disassembly operation. The power cable elbow connector includes a conductor receiving end, loadbreak bushing insert receiving end and a conductive member extending from the cable receiving end to the bushing insert receiving end. The bushing insert receiving end includes an open end portion having an elbow cuff therearound. The loadbreak bushing insert includes an insulative outer housing having an axial bore therethrough and a conductive member positioned within the axial bore. The outer housing includes a power cable elbow insertion end and a mid-section dimensionally radially larger than the power cable elbow insertion end of the outer housing. The outer housing includes a transition shoulder portion between the mid-section and elbow insertion end for providing an interference-fit sealing relationship with the elbow cuff upon insertion of the bushing insert into the power cable elbow. The transition shoulder portion of the bushing insert includes vent means in accordance with the present invention for providing fluid communication between a cavity defined by the elbow cuff and the transition shoulder portion of the bushing insert upon disassembly therebetween and a location outside the mating elbow cuff and transition shoulder portion to prevent a pressure decrease within the cavity and flashover due to a decrease in dielectric strength of the air therein.
The mid-section of the bushing insert includes a conductive portion having least one ground connection terminal thereon for attachment of a ground conductor. In accordance with the present invention, the conductive portion is partially coated with an insulative material between the ground connection terminal and the transition shoulder portion thereby increasing the distance an arc from an energized electrode must travel to ground. Alternatively, the power cable elbow includes a probe or electrode for electrically contacting the conductive member of the bushing insert upon assembly. The probe includes a portion thereof having an insulative material surrounding the probe which extends into the bushing insert upon assembly of the power cable elbow and bushing insert. Accordingly, the distance an arc must travel from the energized electrode to ground is increased by the length of the insulative material surrounding the probe. Furthermore, the power cable elbow includes a conductive insert at the upper end of the bushing insert receiving space. The conductive insert may include insulative material at the upper portion of the bushing insert receiving space to provide an increased distance between an energized electrode and ground.